Oldest macOS for my MacBook, still in service

3 OSes back is the rule of thumb for official support, that'd be Sonoma>Ventura>Monterey.

And I'd suspect Monterey to lose support in early June, as a new OS is released.

But what is your concern with support, I'm running Mojave most of the time, which is 6 OSes ago.

Monterey…

macOS Monterey is compatible with these computers - Apple Support

iMac – all models from late 2015, iMac 16,1 – 20,2

iMac Pro – all models, iMac Pro 1,1

MacBook & MacBook Retina – all models from early 2016, MacBook 9,1 – 10,1

MacBook Pro – all models from early 2015, MacBook Pro 12,1 – 17,1

MacBookAir – all models from early 2015, MacBook Air 7,1 – 10,1

Mac Mini – all models from 2014, Mac Mini 7,1 – 9,1

Mac Pro – all models from 2013, Mac Pro 6,1 – 7,1

The following Mac computers are supported by macOS Ventura:

MacBook (2017 and newer)

MacBook Air (2018 and newer)

MacBook Pro (2017 and newer)

Mac mini (2018 and newer)

Mac Studio (2022)

iMac (2017 and newer)

iMac Pro (2017)

Mac Pro (2019 and newer)

While there may still be a few common third party browsers that will still work with an older OS, this is going to be an issue as well since last year most of the popular third party web browsers announced they will only be supporting the current OS & the two previous versions of macOS just like what Apple officially supports. Some browsers with an ESR version (Extended Support Release) will maintain support until the ESR support expires. Sometimes someone will maintain a special browser with some updates being backported for use with older operating systems in order to give basic browser functionality for older systems.

Any apps which require access to the Internet or other online services will sooner or later become incompatible as the Internet or those services are upgraded over time.

Apps which do not require access to the Internet will remain fully functional except when trying to access files that have been created by newer versions of those apps which may use some new feature. For example, an older Mac running an older OS such as Catalina is not able to view .heic photos which are the default photo format for more recent iPhones/iPads, although you may be able to have the iPhone/iPad convert them to .jpg file or utilize a third party app to convert them.

FYI, here is an article which lists which versions of macOS are compatible with various Apple hardware (goes back as far as macOS 10.8 Mountain Lion):

https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

Also, here is an Apple article with links to various versions of macOS in case you need a particular version of macOS, plus there is a link for creating a bootable macOS USB installer as well (if you have a 2018-2020 T2 Mac, then make sure to first modify the system security using the Startup Security Utility to allow booting from USB before erasing the internal SSD).

How to download and install macOS - Apple Support

I forgot to add the following yesterday:

You also must be very careful when purchasing a used Mac. Many of them may be locked with a firmware password lock (Intel Macs only), or perhaps Activation Locked by an AppleID, or it may even still be managed by a previous owner if the device was not properly removed from an AppleID or from a business' MDM. This forum is full of posts where users found these things out after purchase....sometimes long after a purchase so a refund was not possible.

If you get a Mac with any of these "locks" or management systems still active, then there is absolutely nothing you can do to get full control of that Mac. Also, if that used Mac somehow becomes accidentally locked, you also will not be able to get any assistance from Apple to remove the lock because you will not have the necessary proof of purchase to prove ownership.

Unfortunately it is not always easy to determine if a Mac is still being managed by a previous owner....it can depend upon how strict the management settings are. On a very tightly controlled Mac, then you will see the management notification as soon as you begin setting up a clean OS if the computer is connected to the Internet. For a less strict management setup, or for a Mac that may not currently be connected to the Internet, the management notification may not appear until hours, days, or weeks later.

It is best to buy a used Mac either from someone you know & trust, or from a known reputable seller. Apple even sells officially refurbished devices which would give you the necessary proof of purchase in case the Mac become locked.

FYI, here is what the previous owner should do before selling a Mac:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

I always recommend people perform their own clean re-install of the OS (involves erasing the disk or "Volume Group" --- depends on whether Intel or M-series Mac --- followed by reinstalling the OS) whenever acquiring a used device even if the previous owner did properly prepare the device for sale. Besides giving you peace of mind that the clean install was properly done, it may also help you to discover a problem with that device during the clean install process.

Let's keep talking Mojave as an example. I want to use iTunes natively.... the highest OS for that is the last pre-Catalina OS, or Mojave. I understand that Mojave is technically out of "support" but does that mean it can't support internet connectivity and Apple Store connectivity? Seems like there is some intermediate state between end of support, and end of internet viability.

maybe Apple macOS | endoflife.date

I use mainly Mojave, posting from it now of course.:)

Internet viability is mostly dependent on the Browser, & Safari wewill be the first one out of the picture, as it's basically tied to the OS.

HWTECH - this is a helpful and thorough answer on the level I was hoping for. You got the gist of the question, and Provided details on ways of thinking about what users need to consider.